Method of molding optical articles

ABSTRACT

A method for press molding preforms to obtain optical articles in which each preform is prepared beforehand to have a diameter larger than that of an optically functional area of the optical article and edges of the preform to be press mold contact with molding surfaces of a pair of molds outside an area of each molding surface corresponding to the optically functional area of the optical article, thereby elongating the lifetime of molds.

This is a continuation of application Ser. No. 08/423,932 filed Apr. 18,1995, now abandoned, which was a continuation application of applicationSer. No. 08/206,943, filed Mar. 7, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing opticalarticles of high precision and high quality by press molding, whichconsiderably lengthens the lifetime of molds being used.

2. Explanation of the Prior Art

Many methods of manufacturing optical articles have been proposed. Anexample among them is disclosed in Japanese patent laid-open publicationHEI 3-50126: a preform having a given shape is placed between a pair ofmolds which have a molding surface precisely shaped in the reverse ofthe desired optical article to form, the preform being heated to atemperature at which it can be reshaped, then being pressed to form adesired optical article, and finally being cooled down to the roomtemperature. An example of a preform for use in the molding is describedin the Japanese patent laid-open publication SHO 60-246231 as a glasscylinder cut to a given length, both the end surfaces being polished toa mirror finished surface as shown in FIG. 11. The Japanese patentlaid-open publication SHO 61-261225 describes another example of apreform, which is a block of glass shaped to a sphere as shown in FIG.12.

In the meanwhile, the lifetime of molds is a very important factor inthe press molding of optical articles. Namely, the cost formanufacturing molds becomes very high since they need a very highprecision finishing to mold optical articles of a high precision. Theprice of each optical article press molded includes a fraction of themanufacturing cost of molds naturally. Thus, the cost per opticalarticle to be charged becomes lower as the lifetime of molds becomeslonger.

Conventionally, special cares have been paid for the hardness of apreform material and protection layer formed on a molding surface toenhance the lifetime of molds. Since the molding surface contacts with apreform heated up to a high temperature, it must be covered by aprotection layer which is hardly oxidized and inactive to the preformmaterial such as glass.

Material for molds must have a hardness enough for maintaining a highworking precision against forces exerted from the preform. As to thematerial for molds, cemented carbides or metals including tungsten areused satisfactorily and platinum film is used for the protection layer.The high precision press molding of optical articles becomes availableby development of material for molds and protection layer mentionedabove.

The mold having a protection layer is gradually deteriorated in itsshape precision by use for a long time and the surface of the protectionlayer is roughened by continual contact with glass of a hightemperature. Optical articles thus press molded are deteriorated in theshape precision gradually and, finally, become impossible to satisfy thespecification thereof. In actual use, the lifetime of molds is estimatedso at a time a little bit before optical articles become unsatisfactoryand is indicated by the number of good optical articles produciblethereby.

Conventionally, it has been considered that one of the main factorsdominating the lifetime of molds is deterioration in shape of molds.Accordingly, trials for lengthening the lifetime of molds and, thereby,reducing the price of optical articles are directed to development ofnew materials for molds.

Inventors of the present invention found out the fact that the lifetimeof molds was dominated by relationship between a shape of a moldingsurface and that of a preform. This will be explained in detail below.

Consider a press molding of cylindrical glass preforms as shown in FIG.11.

At the first stage of the press molding, top and bottom molds 11 and 12having molding surfaces 11a and 12a, respectively, contact with circularedges A and B of a cylindrical preform 14 at first. At this stage, apress force against the preform is maintained at a small value to avoidthe preform from breaking or cracking. As the molding progresses withincrease of contact areas between the molding surface and the preform,the press force is gradually increased and, at the final stage, a fullpress force is applied to deform the preform as a whole. At the firststage, edges A and B abut to the same portions of molds always. Thus,concentrated stresses are repeatedly applied to the same portions ofmolds resulting in earlier deterioration of the same portions. In fact,minute concave defects are caused at the specific portion of the moldingsurface and they are transferred as minute convex defects to a surfaceof an optical article upon press molding. Thus, the lifetime of molds isdetermined by generation of these defects substantially.

In the case as shown in FIG. 12, point C on the molding surface receivesa concentrated stress repeatedly and, accordingly, a defect is causedthereat.

SUMMARY OF THE INVENTION

One may solve the above described problem of molding by placing apreform between top and bottom molds so that the preform is in contactwith the molding surfaces only out of their optically functional areas.The preform is then heated and pressed to form a desirable shape. Thepreform, therefore, must be formed so that it is in contact with themolding surfaces only outside their optically functional areas.

The press process may be carried out in error before the preform isheated to a sufficiently high temperature at which it has a low enoughviscosity and can change its shape. In this case the preform wouldcreate flaws and deformation on the molding surfaces by the concentratedstresses generated at the contacts with the molding surfaces. Inemploying the proposed molding method in the present invention, however,the defects would remain out of the optically functional areas, and theywould not adversely affect the performance of optical articles thusformed. Further, the molds maintain a long lifetime despite the defectsthus created.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings throughout which like parts are designated by like referencenumerals, and in which:

FIG. 1 is a schematical cross-sectional view of a main portion of apress molding machine according to the present invention;

FIG. 2 is a perspective view of a preform;

FIG. 3 is a schematical cross-sectional view for showing the first stageof press molding;

FIG. 4 is a perspective view of a bottom mold for showing defects causedon a press molding surface thereof;

FIG. 5 is a perspective view of a convex lens for showing defectstransferred thereto;

FIG. 6 is a front view of another preform to be press molded;

FIG. 7 is a schematical cross-sectional view of a main portion of apress molding machine according to the present invention;

FIG. 8 is a schematical cross-sectional view of a main portion of apress molding machine according to the present invention;

FIG. 9 is a perspective view of a preform to be molded by the press moldshown in FIG. 8;

FIG. 10 is a schematical cross-sectional view of a main portion of apress molding machine according to the present invention;

FIG. 11 is a schematical cross-sectional view of a main portion of aconventional press molding machine; and

FIG. 12 is a schematical cross-sectional view similar to FIG. 11 to showa spherical preform to be press molded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

EMBODIMENT 1

The first embodiment of the present invention will be described withreference to FIG. 1 to FIG. 3. In these figures reference numeral larepresents a preform, 2a a top mold, 2b a bottom mold, and 3 a guidecylinder or guide.

Top and bottom molds 2a and 2b have a spherical molding surface 4machined and processed with precision and have a radius R of curvatureof 4.0 mm, a depth ΔH of 3.0 mm, a diameter (working diameter) D_(w) ofthe concave of 7.7 mm and a diameter D_(m) of the mold of 10.0 mm asshown in FIG. 1. The molding surface 4 consists of an inner area 5 of adiameter D_(eff) of 6.4 mm which press molds an optically functionalportion of an optical article and an outer area 6 which press molds anoptically non-functional portion of the optical article. Normally aprotection layer is formed on the entire area of the molding surfaceincluding both optically functional and non-functional areas, althoughit is not shown in FIG. 1.

As used herein, the expressions "optically functional area and opticallynon-functional area" mean the following: A biconvex lens formed by thismethod of press molding has smooth surfaces which are transferred fromthe entire area of the molding surfaces. The lens is used for imaging orcollecting light. These optical functions are achieved by the lightwhich passes through the area of lens surfaces which are transferredfrom the optically functional area of the molding surfaces. Therefore,the optically functional area is not allowed to contain any flaw ordeformation. Light may be blocked by means of an iris diaphragm to beincident on the areas of the lens surfaces which are transferred fromthe optically non-functional area of the molding surface. Therefore,this area is not as critical as the optically functional area in termsof the optical performance of the lens. Normally both the opticallyfunctional and non-functional areas are processed in the same precision,hence have the same surface smoothness and precision in shape. Theoptically functional area, however, needs an extremely careful attentionto ensure a good surface accuracy and quality.

The inside surface of the guide 3 is finely processed so that the topand bottom molds smoothly slide inside guide 3 along the common centeraxis. The inner diameter of the guide is 10 mm as shown in FIG. 1. Thematerial of molds and guide is a cemented carbide. Preform 1a as shownin FIG. 2 is a cylinder of 7.0 mm in diameter and 6.4 mm in height. Thediameter was determined so that when the preform is placed between topand bottom molds 2a and 2b, the edges of the end surfaces of thecylinder are in contact with the molding surfaces outside the opticallyfunctional area of which diameter is 6.4 mm. The material of the preformis SF-8 (lead glass, the glass transition temperature: 420° C., and thecoefficient of linear thermal expansion: 90×10-7/°C. between 100° C. and300° C.)

FIG. 3 shows an arrangement in which preform 1a is placed in a cavityformed by molds 2a, 2b, and guide 3. Preform 1a was set in the center ofthe cavity so that the edges of the end faces of the preform cylinderare in contact with the molding surfaces within the opticallynon-functional area. The mold assembly with the preform in it was heatedup to a relatively low temperature of 480° C. and then the preform waspressed at a high pressure of 2.5 kgf/mm². They were subsequently cooleddown to the room temperature, and the formed lens was removed from themold assembly. In this process conditions the preform 1a has arelatively high viscosity. After 100 press moldings the molding surfacesof top and bottom molds 2a and 2b were examined under a microscope, anda ring of deep flaws having a width of about 0.2 mm and a centerdiameter D_(o) 7.0 mm was observed on each molding surface, asillustrated in FIG. 4. This ring was outside the optically functionalarea. The lens also had the same rings of flaws which were transferredfrom the molds, as illustrated in FIG. 5. The center diameter D_(o) ofthe ring generated on the molding surfaces coincide with that of preformcylinder 1a. The flaws are, therefore, considered to have been createdby the concentrated stress on the molding surfaces against the edges ofthe end faces of preform 1a. Being limited to outside the opticallyfunctional area, the flaws of lens did not adversely affect itsperformance, and the molds having this kind of flaws were kept beingused after this examination to form lenses of good optical performance.The wave front distortion of the lens measured with a Fizeau typeinterferometer was 1/4 to 1/5 (1=633 nm) as a P-V value. Although inthis embodiment flaws are generated on the molding surfaces, they do notreduce the lifetime of the molds. In fact the lifetime of the mold was afew tens of thousands shots.

For comparison a different preform from the one mentioned above was usedwith the same molds 2a, 2b, and guide 3. This preform was also acylinder, being 9.4 mm in height and 5.8 mm in a diameter which is lessthan that of the optically functional area. It was placed at the centerof the mold assembly cavity and subjected to press molding in the sameconditions as before. In this case the preform was always in contactwith the molding surfaces inside the optically functional area. After100 shots molds 2a and 2b were examined under a microscope and a ring ofdeep flaws was found at a center diameter of about 5.8 mm of eachmolding surface, which is inside the optically functional area.Similarly to the previous case the diameter coincides with that of thepreform, and the flaws are considered to have been caused by theconcentrated stress on the molding surfaces against the edges of the endfaces of the preform. The flaws exist within the optically functionalarea and the formed lenses exhibited poor optical performance and werenot usable in practical applications. In fact measurement of wave frontdistortion was impossible. Therefore, the molds were not usableafterwards and the lifetime must be shorter than 100 shots.

EMBODIMENT 2

The second embodiment of the present invention will be described withreference to FIGS. 6 and 7. Reference numeral 1c represents a preform;2a and 2b top and bottom molds, and 3 a guide which are the same asthose used in embodiment 1.

Preform 1c is a cylinder which has convex end surfaces as shown in FIG.6. The diameter D_(P) of preform 1c is 7.0 mm and the top and bottomconvex surfaces have a radius R of curvature of 10 mm which is largerthan that of the molding surfaces of molds 2a and 2b. Therefore, whenthe preform is placed in the middle between molds 2a and 2b, the preformis in contact with molds 2a and 2b only outside the optically functionalarea having a diameter D_(eff) of 6.4 mm. The height H of the preform is7.8 mm. The material of the preform is optical glass SF-8 which is thesame as the one in embodiment 1. (lead glass, the glass transitiontemperature: 420° C., and the coefficient of linear thermal expansion:90×10⁻⁷ /° C. between 100° C. and 300° C.)

FIG. 7 shows an arrangement in which preform 1c is placed in the cavityformed by molds 2a, 2b, and guide 3. As shown in FIG. 7 preform 1c wasplaced in the center of the cavity so that the edges of the end faces ofthe preform are in contact with the molding surfaces in the opticallynon-functional area. The mold assembly with the preform in it was heatedup to a relatively low temperature of 470° C. and then the preform waspressed at a high pressure of 2.6 kgf/mm². After 100 press moldings themolding surfaces of top and bottom molds 2a and 2b were examined under amicroscope, and a ring of deep flaws of about 6.9 mm in center diameterwas observed on the molding surface of top mold 2a. The lenses thusformed had the same ring of flaws which was transferred from the mold.The center diameter of the ring formed on the molding surface coincideswith that of cylinder 1c. The ring of flaws, therefore, is considered tohave been formed by the concentrated stress on the molding surfaceagainst the edge of the end face of preform 1c. Being limited to outsidethe optically functional area, the flaws did not adversely affect theperformance of the lens, which was approximately the same as that ofembodiment 1. The molds having this kind of flaws were kept being usedafter this examination to form lenses of good optical performance. Infact the lifetime of the mold was a few tens of thousands shots.

EMBODIMENT 3

The third embodiment of the present invention will be described withreference to FIGS. 8, 9 and 10.

As shown in FIG. 8, top mold 2d has a molding surface 5d having a radiusR1 of curvature of 6.8 mm and bottom mold 2b has a molding surface 5bhaving a radius R2 of curvature of 4.0 mm. The optically functionaldiameter D_(eff) (L) of top mold 2d is 6.7 mm and that D_(eff) (S) ofbottom mold 2b is 6.4 mm.

Preform 1a to be press molded is cylindrical and has a diameter D_(P) of7.0 mm and the height H of 6.4 mm.

In this embodiment 3, it is important that diameter D_(P) of preform 1ais larger than optically functional diameter D_(eff) (L) of top mold 2dwhich is larger than D_(eff) (S) of bottom mold 2b.

Thus, circular edges of preform 1a contact with top and bottom molds 2dand 2b outer the optically functional areas, respectively, as shown inFIG. 10.

This guarantees a long lifetime of molds similarly to embodiments 1 and2.

In embodiments 1, 2 and 3 glass was used as an example of a material forpreforms. The shapes of the preforms and the method of molding presentedherein are also applied to other materials. In the embodiments describedabove the preform is in contact with the molding surfaces in a circle atthe beginning of the press molding. This invention, however, is notlimited to a case in which a preform is in contact with molding surfacesin a circle. In fact any figure is allowed. For example, a rectangle anda part of a circle will be as good as a circle. The essence of thepresent invention is that so as to considerably lengthen the lifetime ofthe molds a preform is made in an appropriate shape and size so that thepart of the molds which the preform is in contact with and theconcentration of stress take place at and remain out of the opticallyfunctional area of the molds and that the preform is placed at anappropriate position with regard to molds.

Further it is to be noted that the heating process can be omitted if thepreform is made of a material easy to press mold or it is softenedbefore charging the same between a pair of molds.

Although present invention has been fully described in connection withthe preferred embodiments thereof with reference to the accompanyingdrawings, it is to be noted that various changes and modifications areapparent to those skilled in the art. Such changes and modifications areto be understood as included within the scope of the present inventionas defined by the appended claims unless they depart therefrom.

What is claimed is:
 1. A method of manufacturing an optical article,comprising optical curvatures on both sides of said optical article andsaid optical curvatures comprising optically functional areas ofoptically functional diameters which are less than diameters of theoptical curvatures, comprising the steps of:preparing a cylindricalpreform to be molded using a pair of molds comprising molding curvaturesof working diameters which are equal to respective diameters of saidoptical curvatures of said optical article, said optically functionaldiameters being smaller than said working diameters, the cylindricalpreform having a diameter that is less than a larger one of said workingdiameters and larger than a larger one of said optically functionaldiameters of said optical article; and placing said cylindrical preformbetween said molding curvatures of said pair of molds; and pressing saidcylindrical preform with said molds so that the shapes of said moldingcurvatures are transferred to said cylindrical preform.
 2. The method ofmanufacturing the optical article according to claim 1, wherein eachmold of said pair of molds comprises a concave molding curvature to moldsaid optical article such that the optical curvatures of the opticalarticle are in the shape of convex lenses and said cylindrical preformhas a columnar shape having a larger diameter than said opticallyfunctional diameters of said optically functional areas of said opticalarticle, said cylindrical preform being placed between said concavemolding curvatures with end surfaces of said cylindrical preform facingsaid concave molding curvatures.
 3. The method of manufacturing theoptical article according to claim 2, wherein said columnar shape is acircular cylinder.
 4. The method of manufacturing the optical articleaccording to claim 1, wherein said optical article comprises a firstoptically functional area on one side and a second optically functionalarea on a different side thereof, said first optically functional areahaving a different optically functional diameter than said secondoptically functional area, and said cylindrical preform having adiameter larger than the larger optically functional diameter of saidfirst and second optically functional areas.
 5. The method ofmanufacturing the optical article according to claim 1, furthercomprising a step of heating said preform to soften the same before thestep of pressing said preform.